The first three years of establishment is the most critical time for watering trees. Trees grow best if the rootball and surrounding area remains moist but not soggy. During the first year water weekly to saturate the rootball and surrounding area to a depth of 10-12". Our Extension Master Gardeners put Gator bags around newly planted trees and fill them with 14 gallons of water each week as well as fill the water ring with water. A long screwdriver or piece of rebar should easily go into the soil to the full moistened depth. Water again each week if adequate rainfall is not occurring. During the next two years, soak the rootball and surrounding area every two weeks during drought periods. Continuously saturated wet soils can also be deadly to trees. Let the soil dry out between waterings.
Provide supplemental water to established trees every two weeks during drought periods by soaking the entire root zone (this often goes out to twice the branch spread) to a depth of 10-12". Water slowly by setting a sprinkler in the root zone or set out a soaker hose or a hose with water trickling out the size of a pencil. Let it soak for several hours, or until it has wet the soil to a depth of 10-12".
Woodchip mulches around young trees has the multiple benefits of holding soil moisture, reducing soil temperature and reducing weed growth.